JP5443504B2 - Method for providing drive transistor control signal to drive transistor - Google Patents

Description

  The present invention relates to solid state electroluminescent (EL) flat panel displays, such as organic light emitting diode (OLED) displays, and more particularly has a method for compensating for aging of electroluminescent display components. , Relating to such a display.

  Electroluminescent (EL) devices have been known for many years and have recently been used in commercial display devices. Such devices utilize both active and passive matrix control schemes and can utilize multiple subpixels. Each subpixel includes an EL emitter and a driving transistor for driving a current flowing through the EL emitter. The subpixels are typically arranged in a two-dimensional array, with one row and column address for each subpixel, and one data value associated with the subpixel. The sub-pixels of different colors such as red, green, blue and white are grouped to form a pixel. EL displays can be made from a variety of emitter technologies, including coatable inorganic light emitting diodes, and organic light emitting diodes (OLEDs).

  OLED displays are of particular interest as an excellent flat panel display technology. These displays generate light by using a current flowing through a thin film of organic material. The color of emitted light and the energy conversion efficiency from current to light are determined by the composition of the organic thin film material. Different organic materials emit different colors of light. However, as the display is used, the aging of the organic material within the display occurs and the efficiency in emitting light decreases. This shortens the lifetime of the display. Different organic materials can age at different rates, so as the display is used, the color will age differently and change the white point of the display. Furthermore, each individual pixel undergoes aging at a different rate than the other pixels, resulting in a non-uniform display. In addition, some circuit elements, such as amorphous silicon, are known to exhibit the effects of aging.

  The rate at which aging occurs in a material is related to the amount of current that passes through the display and is therefore related to the amount of light emitted from the display. Various techniques have been described to compensate for this aging effect.

  U.S. Pat. No. 6,057,049 to Shen et al. Calculates the luminous efficiency of individual organic light emitting diodes (OLEDs) in an OLED display by calculating and predicting the decrease in light output efficiency of each pixel based on the cumulative drive current applied to the pixel. Describes a method and associated system for compensating for long-term fluctuations in The method derives a correction factor that is applied to the next drive current for each pixel. This technique requires that the drive current applied to each pixel be measured and accumulated, requires a storage memory that must be constantly updated as the display is used, and is therefore complex and large. Requires a large circuit section.

  U.S. Pat. No. 6,057,096 by Narita et al. Describes a similar method for keeping the amount of light emitted from each light emitting element constant. This design requires the use of a computation unit that responds to each signal transmitted to each pixel to record usage, greatly increasing the complexity of the circuit design.

  U.S. Pat. No. 5,677,096 to Evertt describes a pulse width modulation driver for an OLED display. One embodiment of a video display comprises a voltage driver for applying a selected voltage to drive an organic light emitting diode in the video display. The voltage driver can receive voltage information from the correction table that takes into account aging, column resistance, row resistance, and other diode characteristics. In one embodiment of the invention, the correction table is calculated before and / or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to the OLED current, the correction scheme will deliver a known current into the OLED diode for a long enough duration that the transient can settle, and then , Based on measuring the corresponding voltage using an analog / digital converter (A / D) present on the column driver. The calibration current source and A / D can be switched to any column through the switching matrix.

  Patent Document 4 by Numao describes a method in which the current flowing through an organic EL element and the temperature of the organic EL element are measured. Thereafter, compensation is performed using pre-calculated tables and current and temperature measurements. This design estimates the predictable relative usage of pixels and does not correspond to the difference in actual usage of groups of pixels or individual pixels. Therefore, corrections for color and space groups can become inaccurate over time. Furthermore, it is necessary to integrate a temperature detection circuit and a plurality of current detection circuits in the display. This integration is complex, reduces manufacturing yield and occupies space in the display.

  Japanese Patent Application Laid-Open No. 2005-228867 by Ishizuki et al. Discloses a method for measuring the current for each subpixel in order. The measurement technique of this method is repeated and takes time.

  U.S. Pat. No. 6,057,097 by Arnold et al. Teaches a method for compensating for aging of OLED emitters. This method assumes that the overall change in device luminance is caused by a change in the OLED emitter. However, when the driving transistor in the circuit is formed from amorphous silicon (a-Si), this assumption is not valid because the threshold voltage of the transistor also changes with use. This method does not fully compensate for OLED efficiency loss in circuits where the transistor shows the effects of aging. Further, when using methods such as reverse bias to mitigate a-Si transistor threshold voltage shift, proper tracking / prediction of the reverse bias effect or direct OLED voltage change or transistor threshold voltage change directly. Without using this measurement, compensation for OLED efficiency loss may be unreliable.

  U.S. Patent No. 6,053,077 to Fruehauf discloses a pixel structure having a third transistor that taps a diode drive current for supply to a current measurement circuit and voltage comparison unit. However, this method reduces the efficiency of displays including such pixels by using currents that can be used for light emission if not used for measurements. Furthermore, this method only compensates for TFT variations and cannot compensate for non-uniform OLED characteristics.

  In addition to the effects of aging, drive transistors can be manufactured by some transistor technologies, such as low temperature polysilicon (LTPS), in which the mobility and threshold voltage over the surface of the display. Changes (Non-Patent Document 1). This is undesirable because it creates visible non-uniformities. Furthermore, non-uniform deposition of OLED material is undesirable because it produces emitters with varying efficiencies, which can likewise cause non-uniformities. These non-uniformities are called initial non-uniformities because they exist at the time the panel is sold to the end user. FIG. 9 shows an example histogram of sub-pixel luminance for the plateau showing the characteristic differences between the pixels. In either direction, the actual luminance changed by 20%, resulting in unacceptable display performance.

  U.S. Pat. No. 6,053,086 to Salam describes a display matrix having a process control circuit for reducing brightness fluctuations within a pixel. This disclosure describes using a linear scaling method for each pixel based on the ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will ultimately reduce the dynamic range and brightness of the display as well as reduce and vary the bit depth at which the pixel can be operated.

  U.S. Pat. No. 6,057,091 to Fan describes a method for improving the display uniformity of an OLED. The display characteristics of all organic light emitting devices are measured. The technique implements uniformity correction using a combination of look-up tables and computing circuitry. However, this method requires optical measurements. Thus, this method is not suitable for aging correction that requires periodic measurements at the user's location. In addition, the described approach requires a separate lookup table for each pixel, which can result in being very expensive to meet memory requirements or need to approximate the characteristics of each pixel So either the image quality is degraded.

  Patent document 10 by Kasai et al. Describes an electro-optical device that stabilizes display quality by executing correction processing corresponding to a plurality of disturbance factors and using a conversion table whose description contents include correction coefficients. However, this method requires a large number of lookup tables (LUTs), not all of which are used to perform the process, and does not describe a method for implementing those LUTs. .

  Therefore, there is a need for a more complete compensation approach for aging and initial non-uniformity of electroluminescent displays.

US Pat. No. 6,414,661 US Pat. No. 6,504,565 US Patent Application Publication No. 2002/0167474 JP 2002-278514 A US Patent Application Publication No. 2003/0122813 US Pat. No. 6,995,519 US Patent Application Publication No. 2004/0100430 US Pat. No. 6,081,073 US Pat. No. 6,473,065 US Patent Application Publication No. 2005/0007392

Kuo, Yue, “Thin Film Transistor: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors” (Boston: Kluwer Academic Publishers, 2004, pg. 410-412)

  Therefore, it is an object of the present invention to compensate for aging and efficiency changes in an electroluminescent emitter when the transistor is aging.

The purpose is to provide a drive transistor control signal to a drive transistor in a plurality of electroluminescent (EL) subpixels, comprising:
(A) Providing a plurality of EL subpixels, each subpixel having a driving transistor having a first electrode, a second electrode and a gate electrode, and an EL having a first electrode and a second electrode Comprising an emitter and a read transistor having a first electrode, a second electrode and a gate electrode;
(B) connecting the first electrode of each read transistor to the corresponding second electrode of the drive transistor and the corresponding first electrode of the EL emitter;
(C) for each subpixel, receiving an input code value commanding a corresponding output from each of the subpixels;
(D) selecting a target sub-pixel;
(E) providing each of the sub-pixels excluding the target sub-pixel with the respective input code value, and providing the target sub-pixel with an output that is higher than the corresponding input code value by a selected first amount. Give boost code value to command,
(F) After a selected delay time, on the second electrode of the read transistor of the target subpixel to provide a status signal representative of the characteristics of the drive transistor and the EL emitter in the target subpixel. Measuring a read voltage, wherein the target subpixel is driven with a selected test voltage, the test voltage being provided as a voltage sequence across a threshold voltage of a drive transistor;
(G) using the status signal, providing a compensated code value for the target sub-pixel;
(H) providing a drive transistor control signal corresponding to the compensated code value to the drive transistor of the target EL subpixel; and (i) sequentially selecting the plurality of subpixels as the target subpixel, Repeating steps (d) to (h), each driving transistor control signal being applied to the driving transistor in each of the plurality of EL sub-pixels;
Including
The selected delay time is a selected percentage of the selected frame time;
The selected first quantity is achieved by a method that is a percentage of the output commanded by the corresponding input code value and is calculated as the reciprocal of the selected percentage.

  An advantage of the present invention is that in a display where the aging of the circuitry is occurring without the need for large or complex circuitry to accumulate continuous measurements of light emitting device usage or operating time. It is an OLED display that compensates for aging of organic materials. Yet another advantage of the present invention is that the OLED display uses a simple voltage measurement circuit section. A further advantage of the present invention is that by making all voltage measurements, it is more sensitive to changes than the method of measuring current. A further advantage of the present invention is that, along with compensation for OLED changes, compensation for changes in the characteristics of the drive transistor can be performed, thereby providing a complete compensation solution. A further advantage of the present invention is that both aspects of measurement and compensation (OLED and drive transistor) can be accomplished quickly. A further advantage of the present invention is that data can be input and data read using a single select line. A further advantage of the present invention is that the characterization and compensation of drive transistor and OLED changes are specific to a particular device and are not affected by other devices that can be opened or shorted.

1 is a schematic diagram of one embodiment of an electroluminescent (EL) display that can be used in practicing the present invention. 1 is a schematic diagram of one embodiment of an EL subpixel and associated circuitry that may be used in practicing the present invention. 1 is a schematic diagram of a first embodiment of a conversion circuit that can be used in practicing the present invention. It is the schematic of 2nd Embodiment of the conversion circuit which can be used when implementing this invention. It is a figure which shows the influence which the secular change of an OLED emitter has on luminance efficiency. It is a figure which shows the influence which an aging of an OLED emitter or a drive transistor has on a device current. FIG. 6 is a row timing diagram of an embodiment of the method of the present invention. FIG. 6 is a row timing diagram of another embodiment of the method of the present invention. FIG. 6 is a frame timing diagram of an embodiment of the method of the present invention. 3 is a flow diagram of one embodiment of the method of the present invention. 6 is a graph showing the relationship between a change in transistor threshold voltage and a change in OLED voltage. 6 is a graph showing the relationship between OLED efficiency and changes in OLED voltage. 6 is a graph showing the relationship between OLED efficiency, OLED elapsed time, and OLED drive current density. It is a histogram of pixel luminance showing the difference in characteristics between pixels.

  Referring to FIG. 1, a schematic diagram of one embodiment of an electroluminescent (EL) display that can be used in practicing the present invention is shown. The EL display 10 includes an array of a plurality of EL subpixels 60 arranged in rows and columns. The EL display 10 includes a plurality of row selection lines 20, and each row of EL subpixels 60 has one corresponding row selection line 20. The EL display 10 further includes a plurality of readout lines 30, and each column of EL subpixels 60 has one corresponding readout line 30. Although not shown for clarity of illustration, each column of EL subpixels 60 also has a data line, as is well known in the art. The plurality of readout lines 30 are connected to one or more multiplexers 40, thereby enabling signals to be read out in parallel / sequentially from the EL sub-pixels, as will be described later. The multiplexer 40 can be part of the same structure as the EL display 10, or can have a different configuration that can be connected to or disconnected from the EL display 10.

  Referring now to FIG. 2, a schematic diagram of one embodiment of an EL subpixel and associated circuitry that can be used in practicing the present invention is shown. The EL subpixel 60 includes an EL emitter 50, a driving transistor 70, a capacitor 75, a reading transistor 80, and a selection transistor 90. Each of the transistors includes a first electrode, a second electrode, and a gate electrode. The first voltage source 140 is connected to the first electrode of the driving transistor 70. “Connected” means that the elements are directly connected or connected via another component, eg, a switch, diode, another transistor, or the like. The second electrode of the driving transistor 70 is connected to the first electrode of the EL emitter 50, and the second voltage source 150 is connected to the second electrode of the EL emitter 50. A select transistor 90 connects the data line 35 to the gate electrode of the drive transistor 70 and selectively provides data from the data line 35 to the drive transistor 70 as is well known in the art. Each row select line 20 is connected to the gate electrode of the select transistor 90 and the gate electrode of the read transistor 80 in the corresponding row of the EL subpixel 60.

  The first electrode of the read transistor 80 is connected to the second electrode of the drive transistor 70 and is also connected to the first electrode of the EL emitter 50. Each readout line 30 is connected to a second electrode of readout transistor 80 in the corresponding column of EL subpixels 60. The readout line 30 provides a readout voltage to the measurement circuit 170, which measures the readout voltage and provides a status signal that represents the characteristics of the EL subpixel 60.

  A plurality of readout lines 30 may be connected to the measurement circuit 170 through the multiplexer output line 45 and the multiplexer 40 in order to sequentially read the voltage from the second electrode of each readout transistor of the predetermined number of EL subpixels 60. it can. If there are multiple multiplexers 40, each can have its own multiplexer output line 45. In this way, a predetermined number of EL subpixels can be driven simultaneously. Multiple multiplexers allow voltages to be read from the various multiplexers 40 in parallel, while each multiplexer 40 can sequentially read the readout line 30 attached to that multiplexer. In this specification, this is called a parallel / sequential process.

  The measurement circuit 170 includes a conversion circuit 171 and optionally a processor 190 and a memory 195. The conversion circuit 171 receives the read voltage on the multiplexer output line 45 and outputs digital data on the converted data line 93. The conversion circuit 171 preferably exhibits a high input impedance to the multiplexer output line 45. The read voltage measured by the conversion circuit 171 can be equal to the voltage on the second electrode of the read transistor 90 or can be a function of that voltage. For example, the read voltage measurement can be a voltage on the second electrode of the read transistor 90 minus the drain-source voltage of the read transistor and the voltage drop across the multiplexer 40. Digital data can be used as the status signal, or the status signal can be calculated by the processor 190 as described below. The status signal represents the characteristics of the drive transistor and the EL emitter in the EL subpixel 60. The processor 190 receives the digital data on the converted data line 93 and outputs a status signal on the status line 94. The processor 190 can be a CPU, FPGA or ASIC, and can optionally be connected to the memory 195. The memory 195 can be a non-volatile storage device such as flash or EEPROM, or a volatile storage device such as SRAM.

  Compensator 191 receives the status signal on status line 94, receives the input code value on input line 85, and provides the compensated code value on control line 95. Source driver 155 receives the compensated code value and generates a drive transistor control signal on data line 35. Accordingly, the processor 190 can provide compensated data as described later herein during the display process. As is known in the art, the input code value can be provided by a timing controller (not shown). Input code values can be digital or analog and can be linear or non-linear with respect to the indicated luminance. If analog, the input code value can be a voltage, current, or pulse width modulated waveform.

  Source driver 155 may be a digital / analog converter, or a programmable voltage source, a programmable current source, or a pulse width modulated voltage (“digital drive”) or current driver, or another type of source known in the art. A driver can be included.

  The processor 190 and the compensator 191 can be implemented on the same CPU or other hardware. The processor 190 and compensator 191 can cooperate to provide a predetermined data value on the data line 35 during the measurement process to be described herein.

  Referring to FIG. 3A, in the first embodiment, the conversion circuit 171 includes an analog / digital converter 185 for converting the read voltage measurement value on the multiplexer output line 45 into a digital signal. Those digital signals are provided to the processor 190 on the converted data line 93. The conversion circuit 171 can also include a low-pass filter 180. In this embodiment, a predetermined test data value is provided to the data line 35 by the compensator 191 and the corresponding read voltage on the multiplexer output line 45 is measured and used as a status signal.

  Referring to FIG. 3B, in the second embodiment, the conversion circuit 171 includes a voltage comparator 200, which compares the read voltage measurement on the multiplexer output line 45 with the selected reference voltage level. In comparison, a trigger signal is provided on the trigger line 202 indicating whether the read voltage is above or below the selected reference voltage level. The selected reference voltage level is provided by reference voltage source 201. The read voltage measurement value corresponds to the voltage on the read line 30. To receive the read voltage measurement, the test signal generator 203 sequentially applies the selected test voltage sequence to the gate electrode of the drive transistor. The test signal generator 203 can be a ramp generator, in which case the selected test voltage sequence is a sequence that does not increase or decrease. A sequence that does not increase and a sequence that does not decrease cannot be made constant. The test voltage sequence is also provided to the measurement controller 204, which receives the trigger signal from the voltage comparator 200 and the corresponding test voltage from the test signal generator 203, and has converted the corresponding test voltage. The data line 93 is given to the processor. The processor can provide a corresponding test voltage on the status line 95 as a status signal to the compensator. The measurement controller 204 can also provide a corresponding test voltage function, eg, a primary transformation, as a status signal. Since this embodiment does not require an analog / digital converter, it can be implemented at a lower cost than the first embodiment. The test voltage sequence can be provided to the measurement controller 204 as an equivalent digital code value or as another form of mapping to the test voltage. In this embodiment, the test voltage sequence is provided to the data line 35 by a compensator 191 that receives the sequence from the test signal generator 203 on the control line 95 and the read voltage on the multiplexer output line 45 is defined by the reference voltage 201. The point crossing the threshold to be recorded is recorded and used as a status signal.

While the measurement is taking place, the test data value can indicate to emit light from the EL emitter. This is undesirable as it may be visible to the user of the EL display. As is known in the art, the drive transistor 70 has a threshold voltage V _th, which is relatively less than that voltage (or higher than that voltage for P-channels). Since almost no current flows, relatively little light is emitted. The selected reference voltage level can be less than its threshold voltage to prevent light visible to the user from being emitted during the measurement.

When the driving transistor 70 is an amorphous silicon transistor, the threshold value V _th is known to change under aging conditions including actual use conditions. Therefore, by passing a drive current through the EL emitter 50, V _{th of the} drive transistor 70 increases. Therefore, even if the signal on the gate electrode of the drive transistor 70 is constant, the current I _ds gradually decreases, and therefore the brightness of the light emitted by the EL emitter 50 also gradually decreases. The amount of such reduction will depend on the amount of drive transistor 70 used, and therefore the reduction may be different for different drive transistors in the display. This is one type of spatial variation regarding the characteristics of the EL subpixel 60. Such spatial variations cause the ghost of the image itself to always be displayed on the active display due to differences in brightness and color balance in various parts of the display, and frequently displayed images (eg, network logos). The image “burn-in” can be included. In order to prevent such problems, it is desirable to compensate for such changes in threshold voltage. Also, changes in the EL emitter 50 associated with the elapsed period, such as loss of luminance efficiency and an increase in resistance across the EL emitter 50 may occur.

  Referring now to FIG. 4A, a diagram illustrating the effect of aging of the OLED emitter that occurs as current is passed through the OLED emitter on luminance efficiency is shown. The three curves represent the general performance of different light emitters (eg, red, green and blue emitters, respectively) that emit different colors of light, as represented by the luminance output over time or accumulated current. The decrease in luminance between light emitters of different colors can be different. These differences may be due to different aging characteristics of materials used in different color light emitters, or different usage of different color light emitters. Therefore, in conventional use, if the aging is not corrected, the display will become bright and the color of the display, especially the white point, may shift.

  A further type of spatial variation is initial non-uniformity. The operating life of an EL display is the time from when the end user first sees an image on the display to when the display is discarded. Initial non-uniformity is any non-uniformity present at the beginning of the operating life of the display. The present invention can advantageously correct initial non-uniformities by taking measurements before the operating life of the EL display begins. The measurement can be done at the factory as part of the display manufacturing. Further, the measurement can be performed immediately after the user first activates the product including the EL display and immediately before displaying the first image on the display. This will make the end user's first impression of the display positive, as the display will be able to present a high quality image to the end user when the end user first sees it.

Referring to FIG. 4B, a diagram illustrating the effect of the difference in the characteristics of two EL emitters, the difference in the characteristics of two drive transistors, or both on the EL subpixel current is shown. This figure can also represent a similar case of a single EL subpixel before and after aging. The abscissa in FIG. 3 represents the gate voltage in the driving transistor 70. The ordinate is a logarithm where the bottom of the current flowing through the EL emitter 50 is 10. The first EL sub-pixel I-V characteristic 230 and the second EL sub-pixel I-V characteristic 240 are for two different EL sub-pixels 60, or before aging (230) and after aging (240). The IV curve for a single EL subpixel 60 is shown. In the case of the characteristic 240, a voltage larger than that in the case of the characteristic 230 is required to obtain a desired current. That is, the curve is shifted to the right by the amount ΔV. In the case of aging, ΔV is the sum of the change in threshold voltage (ΔV _th , 210) and the change in EL voltage (ΔV _EL , 220) resulting from the change in EL emitter resistance, as shown in the figure. is there. As a result of this change, light emission is non-uniform between sub-pixels having characteristics 230 and 240, respectively. In characteristic 240 rather than characteristic 230, a given gate voltage will control less current and hence less light.

The relationship between the OLED current I _EL (which is also the drain-source current V _ds flowing through the driving transistor), the OLED voltage V _EL , and the threshold voltage V _th at saturation is as follows.

Where W is the TFT channel width, L is the TFT channel length, μ is the TFT mobility, C ₀ is the oxide capacitance per unit area, V _g is the gate voltage, and V _gs is It is the voltage difference between the gate and source of the driving transistor. For simplicity, ignore the dependence of μ on V _gs . Therefore, changes in V _th and V _EL must be corrected to compensate for variations in the characteristics of one or more of the EL sub-pixels 60. However, many measurements can take a very long time. The present invention advantageously reduces measurement time by correcting transistor variations and EL emitter variations in a single measurement.

  Referring to FIG. 5A, and also referring to FIGS. 2 and 3A, there is shown a timing diagram of the first embodiment given earlier of the present invention. Time passes to the right. Two subpixels addressed as (row, column): (1,1) and (1,2) in row 1 and (2,1) and (2,2) in row 2 Timing is shown. This figure shows for the sake of clarity when the rows do not overlap, but in practice the row times overlap as is known in the art and as shown in FIG. 5C. become.

  For each subpixel, the compensator 191 receives a corresponding input code value on the input line 85, and the code value commands the corresponding light output from each subpixel. Shown in the timing diagram of FIG. 5A is an analog data signal from source driver 155 corresponding to the input code value. Beginning with row 1, the target subpixel (1,1) is selected. A boost code value is calculated, and the boost code value commands a light output for the target subpixel that is higher than the input code value by a selected first amount. The boost code value is given to the target subpixel (1,1) in the boost code value period 302, and all other subpixels, here (1,2), are given their corresponding input code values. (Input code value period 301). After the selected delay time 303, the boost code value period 302 for the target subpixel ends and the measurement time 304 begins. During measurement time 304, the target subpixel is driven with the selected test voltage 305 and, as described above, using the analog / digital converter 185, the voltage on the second electrode of the read transistor of the target subpixel. Measurements are made.

  Referring to FIG. 5B, and referring also to FIGS. 2 and 3B, a timing diagram of the second embodiment given earlier of the present invention is shown. The boost code value period 302, the input code value period 301, the selected delay time 303 and the measurement time 304 are as described in FIG. 3A. During the measurement time 304, the target subpixel is driven with a selected test voltage sequence 306 provided by the test signal generator 203 and, as described above, using the comparator 200, on the second electrode of the read transistor. Is measured.

  As shown in FIGS. 5A and 5B, the measurement process is repeated row by row in a selected order. Any number of subpixels can be selected as target subpixels during any selected row time.

  The boost code value period 302 prevents the measurement from being visible by making the light output of the target subpixel equal to the light output of the other subpixel. During the boost code value period, the target subpixel can be driven to a higher output level to balance the on-time. The delay time 303 may be a selected percentage of the selected row time 307. The selected first quantity is then a certain percentage of the output commanded by the corresponding input code value and can be calculated as the reciprocal of the selected ratio. For example, if the delay time 303 is 0.8 (4/5) of the row time 307, the selected first amount is 1 / 0.8 = 5/4 = 1.25. To reduce the available time by 20%, it is necessary to increase the luminance by 25% to produce the same all-optical output (100% output for row time 1 = 1 * 1 = 1; 125% output when row time is 0.8 = 1.25 * 0.8 = 1).

  Referring to FIG. 5C, as is known in the art, in practice the row times overlap at frame time 308 and delay time 303 is a selected percentage of the selected frame time and the frame time. Can be, for example, 16.7 ms (= 1/60 sec). The measurement time 304 may be before the delay time instead of after the delay time 303. FIG. 5C shows the subpixels in column 1 of each row selected as the target subpixel during the first frame and the subpixels in column 2 of each row selected as the target subpixel during the second frame time. Show. During the second frame, the read voltage measurements obtained during the first frame are used by the compensator 191 to provide compensation provided during the compensation code value period 409 for the sub-pixel that was the target in frame 1 Generated code values are generated.

  Referring now to FIG. 5D and referring also to FIG. 2, a block diagram of one embodiment of the method of the present invention is shown. As described above, an input code value is received (step 310), a target subpixel is selected (step 320), and as described above, the input code value and the boost code value are provided to the subpixel (step 330), A measurement of the voltage on the second electrode of the read transistor of the target subpixel is then performed (step 340). Thereafter, a status signal representing the characteristics of the drive transistor and EL emitter in the target pixel is provided (step 350).

  The status signal can represent aging: over time, the variation of the characteristics of the drive transistor and EL emitter in that target subpixel caused by the operation of the drive transistor 70 and EL emitter 50 in the target subpixel 60. . In order to calculate such a status signal, a first read voltage measurement of each subpixel can be made in any of the embodiments of the conversion circuit 171 described above and stored in the memory 195 by the processor 190. it can. This measurement can be performed before the operating life of the EL display. During operation of the EL display, a second read voltage measurement for each sub-pixel can be made and stored in memory 195 at a later time different from the time at which the first read voltage measurement was made. . The first and second read voltage measurements can then be used to calculate a status signal that represents the variation in characteristics of the drive transistor and EL emitter caused by the operation of the drive transistor and EL emitter over time. For example, the status signal can then be calculated as the difference between the second read voltage measurement and the first read voltage measurement, or as a function of the difference, such as a primary transformation.

  The status signal is then provided to a compensator 191 that provides a compensated code value for the target subpixel using the status signal and the input code value (step 360). The operation of the compensator will be further discussed later.

  A drive transistor control signal corresponding to the compensated code value is then provided to the drive transistor of the target EL subpixel. The compensator provides a compensated code value to the source driver 155, and the source driver 155 generates a drive transistor control signal and applies the signal to the gate electrode of the drive transistor 70 via the data line 35 and the selection transistor 80. (Step 370).

  Steps 320 to 370 are then repeated until a plurality of subpixels are selected in turn as target subpixels and respective drive transistor control signals are applied to the respective drive transistors in each of the plurality of EL subpixels (decision step). 380). Once the readout voltage for one subpixel is measured, the corresponding status signal can be stored in memory 195. The compensator 191 can compensate any number of input code values using the stored status signal. Measurements can be taken at regular intervals or at time intervals determined by the display being turned on or off, or by display usage. Since the boost code value 302 prevents the measurement period 304 from being visible to the user, measurements can also be made throughout the lifetime of the display. Subpixels can be selected as target subpixels in any order. In one embodiment, the sub-pixels can be selected from top to bottom and from left to right, or from right to left, according to the row scan order of the display. In another embodiment, target sub-pixels can be selected at random locations within each row to prevent systematic bias due to factors such as temperature gradients.

Returning to FIG. 2, the voltage _Vout is measured (first embodiment) or selected (second embodiment). The voltage V _data is known (first embodiment) or measured (second embodiment). Since little current flows through the read transistor in the high input impedance converter circuit 171, it can be assumed that the voltage V _read , ie, the voltage drop across the read transistor, is constant. Voltages PVDD and CV are selected. Therefore, V _EL can be calculated as follows.
V _EL = (V _out + V _read ) −CV (2)

Variations in the characteristics of the drive transistors and EL devices in the EL subpixel are reflected in the calculated variations in V _EL . Therefore, V _EL can be used as a status signal. Prior to mass production of the EL display 10, one or more representative devices are characterized, and the transistor characteristics (V _th , mobility) and EL device characteristics (resistance, efficiency) corresponding to V _EL for each subpixel. ) Can be generated. Two or more product models can be created. For example, different areas of the display can have different product models. The product model can be stored in a lookup table or used as an algorithm.

In one embodiment that is particularly useful for compensating for initial non-uniformities, a reference status signal level can be selected. This level can be the average, minimum or maximum value of the status signal for all subpixels, or can be another function as will be apparent to those skilled in the art. The compensator can compare each status signal for each pixel to a reference status signal level to determine how much compensation should be applied. This can be useful in compensating for initial non-uniformities where the second read voltage measurement is not available. The compensator can generate a compensated code value using a product model having the measured V _EL value and the selected reference status signal level.

In one embodiment for aging compensation according to the present invention, the difference ΔV _EL between V _EL at the time of the second read voltage measurement and V _EL at the time of the first read voltage measurement is used as the status signal. Since amorphous silicon TFT aging and OLED aging are both proportional to the integrated current flowing through the device over time, a model can be created that correlates ΔV _EL with the transistor ΔV _th and the compensation performed. . FIG. 6 shows an example of the correlation between ΔV _EL on the abscissa and ΔV _th on the ordinate. This correlation can be incorporated into the product model by regression techniques known in the field of statistics. Curve 390 shows one possible spline fit.

In the case of FIG. 2, for transistor and OLED aging, the compensated code value is ΔV _th and the channel length modulation of the drive transistor 70 due to the OLED voltage rise ΔV _EL , which reduces the V _ds of the drive transistor 70. Only the correction needs to be higher than the input code value.

An additional effect of aging compensation is OLED efficiency loss. An example of the relationship between luminance efficiency and ΔV _{EL for} one device is shown in the graph of FIG. By measuring the luminance decrease for a given current and the relationship between ΔV _EL and the luminance decrease, the change in the corrected signal required for the EL emitter 50 to output the nominal luminance can be determined. . This relationship can be incorporated into the product model.

In order to compensate for changes or variations in the characteristics of the EL subpixel 60, the status signal can be used in an equation of the form:
V _comp = V _data + f ₁ (ΔV _EL ) + f ₂ (ΔV _EL ) + f ₃ (ΔV _EL , ΔV _data )
(3)
Where V _comp is the voltage corresponding to the compensated code value required to maintain the desired luminance of the EL subpixel 60, V _data is the voltage corresponding to the input code value, and f ₁ ( ΔV _EL ) is a correction for a change in threshold voltage, f ₂ (ΔV _EL ) is a correction for a change in EL resistance, and f ₃ (ΔV _EL , ΔV _data ) is a correction for a change in EL efficiency. is there. Function f ₃ is further described below. Functions f ₁ , f ₂ and f ₃ are components of the product model. Using this equation, compensator 191 can control EL emitter 60 to achieve a constant luminance output and an extended lifetime at a given luminance. Since this method provides correction for each EL subpixel in the EL display 10, it compensates for spatial variations in the characteristics of the plurality of EL subpixels.

FIG. 8 shows an example model of f ₃ referred to in Equation 3. The efficiency of the OLED emitter depends not only on its elapsed time, represented by the status signal ΔV _EL , but also on the driven level, represented by V _data . FIG. 8 shows the efficiency vs. drive level curve for seven different aging levels. The aging level is specified as “Txx” as is known in the art. Where “xx” is the percent efficiency at the specified test level, in this case 20 mA / cm ² . The compensator 191 can generate a compensated code value in response to the status signal and the input code value to accurately compensate for variations in the efficiency of the EL emitter at any drive level.

  In a preferred embodiment, the present invention includes, but is not limited to, small molecule or polymer OLEDs as disclosed in US Pat. No. 4,769,292 by Tang et al. And US Pat. No. 5,061,569 by VanSlyke et al. In a display including an organic light emitting diode (OLED). Many combinations and variations of organic light emitting displays can be used to produce such displays. Referring to FIG. 2, when the EL emitter 50 is an OLED emitter, the EL subpixel 60 is an OLED subpixel.

  Transistors 70, 80, and 90 can be amorphous silicon (a-Si) transistors, low temperature polysilicon (LTPS) transistors, zinc oxide transistors, or other transistor types known in the art. The transistors can be N-channel, P-channel, or any combination. The OLED can be a non-inverting structure (as shown) or an inverting structure in which the EL emitter 50 is connected between the first voltage source 140 and the driving transistor 70.

10 EL Display 20 Selection Line 30 Read Line 35 Data Line 40 Multiplexer 45 Multiplexer Output Line 50 EL Device 60 EL Subpixel 70 Drive Transistor 75 Capacitor 80 Read Transistor 85 Input Line 90 Select Transistor 93 Converted Data Line 94 Status Line 95 Control Line 140 First voltage source 150 Second voltage source 155 Source driver 170 Measurement circuit 171 Conversion circuit 180 Low-pass filter 185 Analog / digital converter 190 Processor 191 Compensator 195 Memory 200 Voltage comparator 201 Reference voltage source 202 Trigger line 203 Test signal Generator 204 Measurement controller 210 ΔV _th
220 ΔV _EL
230 Subpixel IV characteristics 240 Subpixel IV characteristics 301 Input code value period 302 Boost code value period 303 Delay time 304 Measurement time 305 Test voltage 306 Test voltage sequence 307 Row time 308 Frame time 310 Step 320 Step 330 Step 340 Step 350 Step 360 Step 370 Step 380 Judgment Step 390 Curve 409 Compensation Code Value Period

Claims (9)

A method of providing drive transistor control signals to drive transistors in a plurality of electroluminescent (EL) sub-pixels, comprising:
(A) Providing a plurality of EL subpixels, each subpixel having a driving transistor having a first electrode, a second electrode and a gate electrode, and an EL having a first electrode and a second electrode Comprising an emitter and a read transistor having a first electrode, a second electrode and a gate electrode;
(B) connecting the first electrode of each read transistor to the corresponding second electrode of the drive transistor and the corresponding first electrode of the EL emitter;
(C) for each subpixel, receiving an input code value commanding a corresponding output from each of the subpixels;
(D) selecting a target sub-pixel;
(E) providing each of the sub-pixels excluding the target sub-pixel with the respective input code value, and providing the target sub-pixel with an output that is higher than the corresponding input code value by a selected first amount. Give boost code value to command,
(F) After a selected delay time, on the second electrode of the read transistor of the target subpixel to provide a status signal representative of the characteristics of the drive transistor and the EL emitter in the target subpixel. Measuring a read voltage, wherein the target subpixel is driven with a selected test voltage, the test voltage being provided as a voltage sequence across a threshold voltage of a drive transistor;
(G) using the status signal, providing a compensated code value for the target sub-pixel;
(H) providing a drive transistor control signal corresponding to the compensated code value to the drive transistor of the target EL subpixel; and (i) sequentially selecting the plurality of subpixels as the target subpixel, Repeating steps (d) to (h), each driving transistor control signal being applied to the driving transistor in each of the plurality of EL sub-pixels;
Including
The selected delay time is a selected percentage of the selected frame time;
The method wherein the selected first quantity is a percentage of the output commanded by the corresponding input code value and is calculated as the reciprocal of the selected percentage.   The method of claim 1, wherein the EL emitter is an OLED emitter.   The method of claim 1, wherein the drive transistor is an amorphous silicon transistor. (J) providing a single readout line connected to the second electrode of the readout transistor of all subpixels for providing a readout voltage; and (k) for each EL subpixel, the corresponding Providing a selection line connected to the gate electrode of the read transistor;
The method of claim 1, further comprising:   The step (f) further includes providing an analog / digital converter connected to the second electrode of the read transistor of the target subpixel, the analog / digital converter used to provide an aging signal. The method of claim 1, wherein: The step (f)
i) providing a voltage comparator connected to the second electrode of the read transistor of the target sub-pixel to provide a trigger signal indicating that the read voltage is greater than or equal to a selected reference voltage level;
ii) providing the gate electrode and the test signal generator for providing sequentially a selected test voltage sequence to the measurement controller of the driving transistor, and iii) which receives said trigger signal from said voltage comparator, pairs The method of claim 1, further comprising providing the measurement controller for providing an aging signal to a compensator using a corresponding test voltage.   The status signal represents a change in characteristics of the drive transistor and the EL emitter in the target subpixel caused by operation of the drive transistor and the EL emitter in the target subpixel over time. The method according to 1. The step (f)
i) providing a memory;
ii) storing a first read voltage measurement for each sub-pixel in the memory;
iii) storing a second read voltage measurement for each sub-pixel in the memory; and iv) using the stored first read voltage measurement and second read voltage measurement. Providing the status signal to a compensator;
The method of claim 7, further comprising:   Further comprising selecting a reference status signal level, wherein step (g) includes using the reference status signal level and providing the compensated code value for the target sub-pixel. Item 2. The method according to Item 1.

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